Highly\nGraphitic Mesoporous Fe,N-Doped Carbon Materials for Oxygen Reduction\nElectrochemical Catalysts

Abstract

The synthesis, characterization,\nand electrocatalytic properties of mesoporous carbon materials doped\nwith nitrogen atoms and iron are reported and compared for the catalyzed\nreduction of oxygen gas at fuel cell cathodes. Mixtures of common\nand inexpensive organic precursors, melamine, and formaldehyde were\npyrolyzed in the presence of transition-metal salts (e.g., nitrates)\nwithin a mesoporous silica template to yield mesoporous carbon materials\nwith greater extents of graphitization than those of others prepared\nfrom small-molecule precursors. In particular, Fe,N-doped carbon materials\npossessed high surface areas (∼800 m²/g) and high\nelectrical conductivities (∼19 S/cm), which make them attractive\nfor electrocatalyst applications. The surface compositions of the\nmesoporous Fe,N-doped carbon materials were postsynthetically modified\nby acid washing and followed by high-temperature thermal treatments,\nwhich were shown by X-ray photoelectron spectroscopy to favor the\nformation of graphitic and pyridinic nitrogen moieties. Such surface-modified\nmaterials exhibited high electrocatalytic oxygen reduction activities\nunder alkaline conditions, as established by their high onset and\nhalf-wave potentials (1.04 and 0.87 V, respectively vs reversible\nhydrogen electrode) and low Tafel slope (53 mV/decade). These values\nare superior to many similar transition-metal- and N-doped carbon\nmaterials and compare favorably with commercially available precious-metal\ncatalysts, e.g., 20 wt % Pt supported on activated carbon. The analyses\nindicate that inexpensive mesoporous Fe,N-doped carbon materials are\npromising alternatives to precious metal-containing catalysts for\nelectrochemical reduction of oxygen in polymer electrolyte fuel cells.